Currently, there exist a variety of methods for adjusting a frequency of a piezoelectric vibrator having a plurality of vibration arms.
In a frequency adjustment method disclosed in Patent Document 1 cited below, a piezoelectric vibrator disposed in a package is irradiated with a femtosecond laser. This makes it possible to remove a mass film or a base material of the piezoelectric vibrator so as to adjust the frequency while suppressing damage to the package. A wave length of the laser is defined to be no less than 1200 nm.
In a frequency adjustment method disclosed in Patent Document 2 cited below, a quartz tuning fork vibrator disposed in a package is irradiated with a laser. An adjustment film on a leading end of the quartz tuning fork vibrator scatters when removed by the laser. In order to prevent the adjustment film that has scattered from reattaching to the quartz tuning fork vibrator, a projection is provided on an inner wall of the package.
In addition, there is disclosed a method in which an alteration layer having a different crystal structure from a portion surrounding the stated alteration layer is provided by laser irradiation.
Patent Document 3, cited below, discloses that an alteration layer is provided in a vibration arm of a tuning fork vibrator where a piezoelectric thin film is formed on a quartz or Si substrate by irradiating the vibration arm with a femtosecond laser. To be more specific, the alteration layer is provided within a range of 0.4 to 0.6 times the length of the vibration arm. This suppresses the secondary mode. Note that Patent Document 3 does not disclose a frequency adjustment method.
Patent Document 1: WO 2011/043357.
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2010-118784.
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2011-160391.
With the method in Patent Document 1, there is a case in which a mass film having been removed by the laser scatters within the package and reattaches to the piezoelectric vibrator, thereby causing deviation from the target frequency. Further, when the mass film having reattached is gasified over time, a variation in frequency or the like occurs to make the characteristics likely to be unstable. Furthermore, there is a risk of a short circuit in the case where the mass film is attached between electrode patterns. In addition, in the case where a surface of the piezoelectric vibrator is irradiated with the laser, a residual substance is formed on the periphery. As such, in the case where the residual substance is attached to a section of vibration, the vibration is obstructed and there arises a risk of a Q-value being lowered, especially at a time of large amplitude driving.
With the method in Patent Document 2, providing a projection on the inner wall of the package makes a height of the package larger and raises a problem that a quartz vibration device becomes large in size. In addition, even if the projection is provided on the inner wall of the package, in the case where a gasified adjustment film moves within the package, the stated gasified film cannot be completely prevented from reattaching to the quartz tuning fork vibrator.
With the method in Patent Document 3, providing an alteration layer in the vibration arm is likely to lower the strength of the vibration arm. In particular, as the amplitude of the vibration is larger, the strength is likely to be significantly lowered because repetitive stress becomes larger.